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.\"     @(#)intro.2	8.3 (Berkeley) 12/11/93
.\"
.Dd $Mdocdate: July 28 2015 $
.Dt INTRO 2
.Os
.Sh NAME
.Nm intro
.Nd introduction to system calls and error numbers
.Sh SYNOPSIS
.In errno.h
.Sh DESCRIPTION
The manual pages in section 2 provide an overview of the system calls,
their error returns, and other common definitions and concepts.
.\".Pp
.\".Sy System call restart
.\".Pp
.\"<more later...>
.Sh DIAGNOSTICS
Nearly all of the system calls provide an error number via the identifier
.Dv errno ,
which expands to an addressable location of type
.Vt int .
The address of
.Va errno
in each thread is guaranteed to be unique for the lifetime of the thread.
Applications must use
.Va errno
as defined in
.In errno.h
and not attempt to use a custom definition.
.Pp
When a system call detects an error, it returns an integer value
indicating failure (usually \-1) and sets the variable
.Va errno
accordingly.
(This allows interpretation of the failure on receiving
a \-1 and to take action accordingly.)
Successful calls never set
.Va errno ;
once set, it remains until another error occurs.
It should only be examined after an error.
Note that a number of system calls overload the meanings of these
error numbers, and that the meanings must be interpreted according
to the type and circumstances of the call.
.Pp
The following is a complete list of the errors and their
names as given in
.In sys/errno.h .
.Bl -hang -width Ds
.It Er 0 Em "Undefined error: 0" .
Not used.
.It Er 1 EPERM Em "Operation not permitted" .
An attempt was made to perform an operation limited to processes
with appropriate privileges or to the owner of a file or other
resources.
.It Er 2 ENOENT Em "\&No such file or directory" .
A component of a specified pathname did not exist, or the
pathname was an empty string.
.It Er 3 ESRCH Em "\&No such process" .
No process could be found which corresponds to the given process ID.
.It Er 4 EINTR Em "Interrupted system call" .
An asynchronous signal (such as
.Dv SIGINT
or
.Dv SIGQUIT )
was caught by the thread during the execution of an interruptible
function.
If the signal handler performs a normal return, the
interrupted function call will seem to have returned the error condition.
.It Er 5 EIO Em "Input/output error" .
Some physical input or output error occurred.
This error will not be reported until a subsequent operation on the same file
descriptor and may be lost (overwritten) by any subsequent errors.
.It Er 6 ENXIO Em "Device not configured" .
Input or output on a special file referred to a device that did not
exist, or made a request beyond the limits of the device.
This error may also occur when, for example, a tape drive is not online or
no disk pack is loaded on a drive.
.It Er 7 E2BIG Em "Argument list too long" .
The number of bytes used for the argument and environment
list of the new process exceeded the limit
.Dv ARG_MAX .
.It Er 8 ENOEXEC Em "Exec format error" .
A request was made to execute a file that, although it has the appropriate
permissions, was not in the format required for an executable file.
.It Er 9 EBADF Em "Bad file descriptor" .
A file descriptor argument was out of range, referred to no open file,
or a read (write) request was made to a file that was only open for
writing (reading).
.It Er 10 ECHILD Em "\&No child processes" .
A
.Xr wait 2
or
.Xr waitpid 2
function was executed by a process that had no existing or unwaited-for
child processes.
.It Er 11 EDEADLK Em "Resource deadlock avoided" .
An attempt was made to lock a system resource that
would have resulted in a deadlock situation.
.It Er 12 ENOMEM Em "Cannot allocate memory" .
The new process image required more memory than was allowed by the hardware
or by system-imposed memory management constraints.
A lack of swap space is normally temporary; however, a lack of core is not.
Soft limits may be increased to their corresponding hard limits.
.It Er 13 EACCES Em "Permission denied" .
An attempt was made to access a file in a way forbidden
by its file access permissions.
.It Er 14 EFAULT Em "Bad address" .
The system detected an invalid address in attempting to
use an argument of a call.
.It Er 15 ENOTBLK Em "Block device required" .
A block device operation was attempted on a non-block device or file.
.It Er 16 EBUSY Em "Device busy" .
An attempt to use a system resource which was in use at the time
in a manner which would have conflicted with the request.
.It Er 17 EEXIST Em "File exists" .
An existing file was mentioned in an inappropriate context,
for instance, as the new link name in a
.Xr link 2
function.
.It Er 18 EXDEV Em "Cross-device link" .
A hard link to a file on another file system was attempted.
.It Er 19 ENODEV Em "Operation not supported by device" .
An attempt was made to apply an inappropriate function to a device,
for example, trying to read a write-only device such as a printer.
.It Er 20 ENOTDIR Em "Not a directory" .
A component of the specified pathname existed, but it was
not a directory, when a directory was expected.
.It Er 21 EISDIR Em "Is a directory" .
An attempt was made to open a directory with write mode specified.
.It Er 22 EINVAL Em "Invalid argument" .
Some invalid argument was supplied.
(For example, specifying an undefined signal to a
.Xr signal 3
or
.Xr kill 2
function).
.It Er 23 ENFILE Em "Too many open files in system" .
Maximum number of file descriptors allowable on the system
has been reached and a request for an open cannot be satisfied
until at least one has been closed.
The
.Xr sysctl 3
variable
.Va kern.maxfiles
contains the current limit.
.It Er 24 EMFILE Em "Too many open files" .
The maximum number of file descriptors allowable for this process
has been reached and a request for an open cannot be satisfied
until at least one has been closed.
.Xr getdtablesize 3
will obtain the current limit.
.It Er 25 ENOTTY Em "Inappropriate ioctl for device" .
A control function (see
.Xr ioctl 2 )
was attempted for a file or
special device for which the operation was inappropriate.
.It Er 26 ETXTBSY Em "Text file busy" .
An attempt was made either to execute a pure procedure (shared text)
file which was open for writing by another process,
or to open with write access a pure procedure file that is currently
being executed.
.It Er 27 EFBIG Em "File too large" .
The size of a file exceeded the maximum.
(The system-wide maximum file size is 2**63 bytes.
Each file system may impose a lower limit for files contained within it.)
.It Er 28 ENOSPC Em "\&No space left on device" .
A
.Xr write 2
to an ordinary file, the creation of a directory or symbolic link,
or the creation of a directory entry failed because no more disk
blocks were available on the file system, or the allocation of an
inode for a newly created file failed because no more inodes were
available on the file system.
.It Er 29 ESPIPE Em "Illegal seek" .
An
.Xr lseek 2
function was issued on a socket, pipe or FIFO.
.It Er 30 EROFS Em "Read-only file system" .
An attempt was made to modify a file or create a directory
on a file system that was read-only at the time.
.It Er 31 EMLINK Em "Too many links" .
The maximum allowable number of hard links to a single file has been
exceeded (see
.Xr pathconf 2
for how to obtain this value).
.It Er 32 EPIPE Em "Broken pipe" .
A write on a pipe, socket or FIFO
for which there is no process to read the data.
.It Er 33 EDOM Em "Numerical argument out of domain" .
A numerical input argument was outside the defined domain of
the mathematical function.
.It Er 34 ERANGE Em "Result too large" .
A result of the function was too large to fit in the
available space (perhaps exceeded precision).
.It Er 35 EAGAIN Em "Resource temporarily unavailable" .
This is a temporary condition and later calls to the
same routine may complete normally.
.It Er 36 EINPROGRESS Em "Operation now in progress" .
An operation that takes a long time to complete (such as a
.Xr connect 2 )
was attempted on a non-blocking object (see
.Xr fcntl 2 ) .
.It Er 37 EALREADY Em "Operation already in progress" .
An operation was attempted on a non-blocking object that already
had an operation in progress.
.It Er 38 ENOTSOCK Em "Socket operation on non-socket" .
Self-explanatory.
.It Er 39 EDESTADDRREQ Em "Destination address required" .
A required address was omitted from an operation on a socket.
.It Er 40 EMSGSIZE Em "Message too long" .
A message sent on a socket was larger than the internal message buffer
or some other network limit.
.It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" .
A protocol was specified that does not support the semantics of the
socket type requested.
For example, you cannot use the Internet UDP protocol with type
.Dv SOCK_STREAM .
.It Er 42 ENOPROTOOPT Em "Protocol not available" .
A bad option or level was specified in a
.Xr getsockopt 2
or
.Xr setsockopt 2
call.
.It Er 43 EPROTONOSUPPORT Em "Protocol not supported" .
The protocol has not been configured into the
system or no implementation for it exists.
.It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" .
The support for the socket type has not been configured into the
system or no implementation for it exists.
.It Er 45 EOPNOTSUPP Em "Operation not supported" .
The attempted operation is not supported for the type of object referenced.
Usually this occurs when a file descriptor refers to a file or socket
that cannot support this operation, for example, trying to
.Em accept
a connection on a datagram socket.
.It Er 46 EPFNOSUPPORT Em "Protocol family not supported" .
The protocol family has not been configured into the
system or no implementation for it exists.
.It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" .
An address incompatible with the requested protocol was used.
For example, you shouldn't necessarily expect to be able to use
NS addresses with Internet protocols.
.It Er 48 EADDRINUSE Em "Address already in use" .
Only one usage of each address is normally permitted.
.It Er 49 EADDRNOTAVAIL Em "Can't assign requested address" .
Normally results from an attempt to create a socket with an
address not on this machine.
.It Er 50 ENETDOWN Em "Network is down" .
A socket operation encountered a dead network.
.It Er 51 ENETUNREACH Em "Network is unreachable" .
A socket operation was attempted to an unreachable network.
.It Er 52 ENETRESET Em "Network dropped connection on reset" .
The host you were connected to crashed and rebooted.
.It Er 53 ECONNABORTED Em "Software caused connection abort" .
A connection abort was caused internal to your host machine.
.It Er 54 ECONNRESET Em "Connection reset by peer" .
A connection was forcibly closed by a peer.
This normally results from a loss of the connection on the remote socket
due to a timeout or a reboot.
.It Er 55 ENOBUFS Em "\&No buffer space available" .
An operation on a socket or pipe was not performed because
the system lacked sufficient buffer space or because a queue was full.
.It Er 56 EISCONN Em "Socket is already connected" .
A
.Xr connect 2
request was made on an already connected socket; or, a
.Xr sendto 2
or
.Xr sendmsg 2
request on a connected socket specified a destination
when already connected.
.It Er 57 ENOTCONN Em "Socket is not connected" .
A request to send or receive data was disallowed because
the socket was not connected and (when sending on a datagram socket)
no address was supplied.
.It Er 58 ESHUTDOWN Em "Can't send after socket shutdown" .
A request to send data was disallowed because the socket
had already been shut down with a previous
.Xr shutdown 2
call.
.It Er 59 ETOOMANYREFS Em "Too many references: can't splice" .
Not used in
.Ox .
.It Er 60 ETIMEDOUT Em "Operation timed out" .
A
.Xr connect 2
or
.Xr send 2
request failed because the connected party did not
properly respond after a period of time.
(The timeout period is dependent on the communication protocol.)
.It Er 61 ECONNREFUSED Em "Connection refused" .
No connection could be made because the target machine actively
refused it.
This usually results from trying to connect to a service that is
inactive on the foreign host.
.It Er 62 ELOOP Em "Too many levels of symbolic links" .
A path name lookup involved more than 32
.Pq Dv SYMLOOP_MAX
symbolic links.
.It Er 63 ENAMETOOLONG Em "File name too long" .
A component of a pathname exceeded 255
.Pq Dv NAME_MAX
characters, or an entire pathname (including the terminating NUL)
exceeded 1024
.Pq Dv PATH_MAX
bytes.
.It Er 64 EHOSTDOWN Em "Host is down" .
A socket operation failed because the destination host was down.
.It Er 65 EHOSTUNREACH Em "\&No route to host" .
A socket operation was attempted to an unreachable host.
.It Er 66 ENOTEMPTY Em "Directory not empty" .
A directory with entries other than
.Ql \&.
and
.Ql \&..
was supplied to a remove directory or rename call.
.It Er 67 EPROCLIM Em "Too many processes" .
.It Er 68 EUSERS Em "Too many users" .
The quota system ran out of table entries.
.It Er 69 EDQUOT Em "Disc quota exceeded" .
A
.Xr write 2
to an ordinary file, the creation of a directory or symbolic link,
or the creation of a directory entry failed because the user's quota
of disk blocks was exhausted, or the allocation of an inode for a newly
created file failed because the user's quota of inodes was exhausted.
.It Er 70 ESTALE Em "Stale NFS file handle" .
An attempt was made to access an open file on an NFS filesystem which
is now unavailable as referenced by the file descriptor.
This may indicate the file was deleted on the NFS server or some other
catastrophic event occurred.
.It Er 72 EBADRPC Em "RPC struct is bad" .
Exchange of
.Xr rpc 3
information was unsuccessful.
.It Er 73 ERPCMISMATCH Em "RPC version wrong" .
The version of
.Xr rpc 3
on the remote peer is not compatible with the local version.
.It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" .
The requested
.Xr rpc 3
program is not registered on the remote host.
.It Er 75 EPROGMISMATCH Em "Program version wrong" .
The requested version of the
.Xr rpc 3
program is not available on the remote host.
.It Er 76 EPROCUNAVAIL Em "Bad procedure for program" .
An
.Xr rpc 3
call was attempted for a procedure which doesn't exist
in the remote program.
.It Er 77 ENOLCK Em "\&No locks available" .
A system-imposed limit on the number of simultaneous file
locks was reached.
.It Er 78 ENOSYS Em "Function not implemented" .
Attempted a system call that is not available on this
system.
.It Er 79 EFTYPE Em "Inappropriate file type or format" .
The file contains invalid data or set to invalid modes.
.It Er 80 EAUTH Em "Authentication error" .
Attempted to use an invalid authentication ticket to mount a
NFS filesystem.
.It Er 81 ENEEDAUTH Em "Need authenticator" .
An authentication ticket must be obtained before the given
NFS filesystem may be mounted.
.It Er 82 EIPSEC Em "IPsec processing failure" .
IPsec subsystem error.
Not used in
.Ox .
.It Er 83 ENOATTR Em "Attribute not found" .
A UFS Extended Attribute is not found for the specified pathname.
.It Er 84 EILSEQ Em "Illegal byte sequence" .
An illegal sequence of bytes was used when using wide characters.
.It Er 85 ENOMEDIUM Em "\&No medium found" .
Attempted to use a removable media device with no medium present.
.It Er 86 EMEDIUMTYPE Em "Wrong medium type" .
Attempted to use a removable media device with incorrect or incompatible
medium.
.It Er 87 EOVERFLOW Em "Value too large to be stored in data type" .
A numerical result of the function was too large to be stored in the
caller provided space.
.It Er 88 ECANCELED Em "Operation canceled" .
The requested operation was canceled.
.It Er 89 EIDRM Em "Identifier removed" .
An IPC identifier was removed while the current thread was waiting on it.
.It Er 90 ENOMSG Em "\&No message of desired type".
An IPC message queue does not contain a message of the desired type,
or a message catalog does not contain the requested message.
.It Er 91 ENOTSUP Em "Not supported" .
The operation has requested an unsupported value.
.El
.Sh DEFINITIONS
.Bl -tag -width Ds
.It Process
A process is a collection of one or more threads,
plus the resources shared by those threads such as process ID,
address space,
user IDs and group IDs,
and root directory and current working directory.
.It Process ID
Each active process in the system is uniquely identified by a non-negative
integer called a process ID.
The range of this ID is from 1 to 32766.
.It Parent Process ID
A new process is created by a currently active process; (see
.Xr fork 2 ) .
The parent process ID of a process is initially the process ID of its creator.
If the creating process exits,
the parent process ID of each child is set to the ID of a system process,
.Xr init 8 .
.It Process Group
Each active process is a member of a process group that is identified by
a non-negative integer called the process group ID.
This is the process ID of the group leader.
This grouping permits the signaling of related processes (see
.Xr termios 4 )
and the job control mechanisms of
.Xr csh 1 .
.It Session
A session is a set of one or more process groups.
A session is created by a successful call to
.Xr setsid 2 ,
which causes the caller to become the only member of the only process
group in the new session.
.It Session Leader
A process that has created a new session by a successful call to
.Xr setsid 2 ,
is known as a session leader.
Only a session leader may acquire a terminal as its controlling terminal (see
.Xr termios 4 ) .
.It Controlling Process
A session leader with a controlling terminal is a controlling process.
.It Controlling Terminal
A terminal that is associated with a session is known as the controlling
terminal for that session and its members.
.It Terminal Process Group ID
A terminal may be acquired by a session leader as its controlling terminal.
Once a terminal is associated with a session, any of the process groups
within the session may be placed into the foreground by setting
the terminal process group ID to the ID of the process group.
This facility is used
to arbitrate between multiple jobs contending for the same terminal;
(see
.Xr csh 1
and
.Xr tty 4 ) .
.It Orphaned Process Group
A process group is considered to be
.Em orphaned
if it is not under the control of a job control shell.
More precisely, a process group is orphaned
when none of its members has a parent process that is in the same session
as the group,
but is in a different process group.
Note that when a process exits, the parent process for its children
is changed to be
.Xr init 8 ,
which is in a separate session.
Not all members of an orphaned process group are necessarily orphaned
processes (those whose creating process has exited).
The process group of a session leader is orphaned by definition.
.It Thread
A thread is a preemptively scheduled flow of control within a process,
with its own set of register values,
floating point environment,
thread ID,
signal mask,
pending signal set,
alternate signal stack,
thread control block address,
resource utilization,
errno variable location,
and values for thread-specific keys.
A process initially has just one thread,
a duplicate of the thread in the parent process that created this process.
.It Real User ID and Real Group ID
Each user on the system is identified by a positive integer
termed the real user ID.
.Pp
Each user is also a member of one or more groups.
One of these groups is distinguished from others and
used in implementing accounting facilities.
The positive integer corresponding to this distinguished group is termed
the real group ID.
.Pp
All processes have a real user ID and real group ID.
These are initialized from the equivalent attributes
of the process that created it.
.It "Effective User ID, Effective Group ID, and Group Access List"
Access to system resources is governed by two values:
the effective user ID, and the group access list.
The first member of the group access list is also known as the
effective group ID.
(In POSIX.1, the group access list is known as the set of supplementary
group IDs, and it is unspecified whether the effective group ID is
a member of the list.)
.Pp
The effective user ID and effective group ID are initially the
process's real user ID and real group ID respectively.
Either may be modified through execution of a set-user-ID or set-group-ID
file (possibly by one of its ancestors) (see
.Xr execve 2 ) .
By convention, the effective group ID (the first member of the group access
list) is duplicated, so that the execution of a set-group-ID program
does not result in the loss of the original (real) group ID.
.Pp
The group access list is a set of group IDs
used only in determining resource accessibility.
Access checks are performed as described below in
.Dq File Access Permissions .
.It Saved Set User ID and Saved Set Group ID
When a process executes a new file, the effective user ID is set
to the owner of the file if the file is set-user-ID, and the effective
group ID (first element of the group access list) is set to the group
of the file if the file is set-group-ID.
The effective user ID of the process is then recorded as the saved set-user-ID,
and the effective group ID of the process is recorded as the saved set-group-ID.
These values may be used to regain those values as the effective user
or group ID after reverting to the real ID (see
.Xr setuid 2 ) .
(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional,
and are used in setuid and setgid, but this does not work as desired
for the superuser.)
.It Superuser
A process is recognized as a
.Em superuser
process and is granted special privileges if its effective user ID is 0.
.It Special Processes
The processes with process IDs of 0 and 1 are special.
Process 0 is the scheduler.
Process 1 is the initialization process
.Xr init 8 ,
and is the ancestor of every other process in the system.
It is used to control the process structure.
.It Descriptor
An integer assigned by the system when a file is referenced
by
.Xr open 2
or
.Xr dup 2 ,
or when a socket is created by
.Xr pipe 2 ,
.Xr socket 2
or
.Xr socketpair 2 ,
which uniquely identifies an access path to that file or socket from
a given process or any of its children.
.It File Name
Names consisting of up to 255
.Pq Dv NAME_MAX
characters may be used to name
an ordinary file, special file, or directory.
.Pp
These characters may be arbitrary eight-bit values,
excluding 0 (NUL) and the ASCII code for
.Ql \&/
(slash).
.Pp
Note that it is generally unwise to use
.Ql \&* ,
.Ql \&? ,
.Ql \&[
or
.Ql \&]
as part of
file names because of the special meaning attached to these characters
by the shell.
.Pp
Note also that
.Dv NAME_MAX
is an upper limit fixed by the kernel, meant to be used for sizing buffers.
Some filesystems may have additional restrictions.
These can be queried using
.Xr pathconf 2
and
.Xr fpathconf 2 .
.It Path Name
A path name is a NUL-terminated
character string starting with an
optional slash
.Ql \&/ ,
followed by zero or more directory names separated
by slashes, optionally followed by a file name.
The total length of a path name must be less than 1024
.Pq Dv PATH_MAX
characters.
Additional restrictions may apply, depending upon the filesystem, to be
queried with
.Xr pathconf 2
or
.Xr fpathconf 2
if needed.
.Pp
If a path name begins with a slash, the path search begins at the
.Em root
directory.
Otherwise, the search begins from the current working directory.
A slash by itself names the root directory.
An empty pathname is invalid.
.It Directory
A directory is a special type of file that contains entries
that are references to other files.
Directory entries are called links.
By convention, a directory contains at least two links,
.Ql \&.
and
.Ql \&.. ,
referred to as
.Em dot
and
.Em dot-dot
respectively.
Dot refers to the directory itself and dot-dot refers to its
parent directory.
.It "Root Directory and Current Working Directory"
Each process has associated with it a concept of a root directory
and a current working directory for the purpose of resolving path
name searches.
A process's root directory need not be the root directory of
the root file system.
.It File Access Permissions
Every file in the file system has a set of access permissions.
These permissions are used in determining whether a process
may perform a requested operation on the file (such as opening
a file for writing).
Access permissions are established at the time a file is created.
They may be changed at some later time through the
.Xr chmod 2
call.
.Pp
File access is broken down according to whether a file may be: read,
written, or executed.
Directory files use the execute permission to control if the directory
may be searched.
.Pp
File access permissions are interpreted by the system as
they apply to three different classes of users: the owner
of the file, those users in the file's group, anyone else.
Every file has an independent set of access permissions for
each of these classes.
When an access check is made, the system decides if permission should be
granted by checking the access information applicable to the caller.
.Pp
Read, write, and execute/search permissions on
a file are granted to a process if:
.Pp
The process's effective user ID is that of the superuser.
(Note: even the superuser cannot execute a non-executable file.)
.Pp
The process's effective user ID matches the user ID of the owner
of the file and the owner permissions allow the access.
.Pp
The process's effective user ID does not match the user ID of the
owner of the file, and either the process's effective
group ID matches the group ID
of the file, or the group ID of the file is in
the process's group access list,
and the group permissions allow the access.
.Pp
Neither the effective user ID nor effective group ID
and group access list of the process
match the corresponding user ID and group ID of the file,
but the permissions for
.Dq other users
allow access.
.Pp
Otherwise, permission is denied.
.It Sockets and Address Families
A socket is an endpoint for communication between processes.
Each socket has queues for sending and receiving data.
.Pp
Sockets are typed according to their communications properties.
These properties include whether messages sent and received
at a socket require the name of the partner, whether communication
is reliable, the format used in naming message recipients, etc.
.Pp
Each instance of the system supports some
collection of socket types; consult
.Xr socket 2
for more information about the types available and
their properties.
.Pp
Each instance of the system supports some number of sets of
communications protocols.
Each protocol set supports addresses of a certain format.
An Address Family is the set of addresses for a specific group of protocols.
Each socket has an address chosen from the address family in which the
socket was created.
.El
.Sh SEE ALSO
.Xr intro 3 ,
.Xr perror 3
.Sh HISTORY
An
.Nm
manual page appeared in
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